Engine

ABSTRACT

A 4-cycle, internal combustion engine comprising a crankcase and a pair of cylinders, each cylinder being open at one end into communication with the crankcase. Each cylinder is provided with an inlet port, and a crankshaft is disposed in the crankcase to be adjacent the open ends of the cylinders. A piston is reciprocably received in each cylinder to define therewithin a variable-volume combustion chamber, each piston being drivingly connected in a conventional manner to the crankshaft. A system is provided for admitting a gaseous mixture to the crankcase, and there is a passageway leading from the crankcase to each inlet port. valves are provided for respectively opening and closing the passageways, the valves being drivingly connected to and operated by the crankshaft. The valves are arranged and driven such that each passageway is opened only on alternate occasions when both pistons move toward the axis of the crankshaft. Thus, the pumping action of both pistons drives the gaseous mixture through the only open passageway into one of the two cylinders. The inlet port and an exhaust port is provided in the side wall of each cylinder at peripherally spaced apart points remote from the closed end of the cylinder. An auxiliary chamber in open communication with each exhaust port is provided, each said auxiliary chamber being provided with a port at a point remote from the point of communication of the chamber with the cylinder exhaust port. A butterfly valve is provided for dominating each auxiliary chamber port, the butterfly valve being drivingly connected to the crankshaft.

United States Patent 91 Gommel 51 Sept. 4, 1973 ENGINE [76] Inventor: Dewey E. Gommel, P.O. Box 4627,

Greenville, Miss. 38701 [22] Filed: Jan. 17, 1972 [21] App]. No.: 218,373

[52] US. Cl. 123/75 CC, 123/59 A, 123/73 R [51] Int. Cl. F02b 75/02, F02d 39/02 [58] Field of Search 123/75 CC, 59 A, 123/57, 75, 59, 73

[56] References Cited UNITED STATES PATENTS 1,046,392 12/1912 Kessler 123/75 CC 1,255,150 2/1918 Franklin 123/77 CC 1,267,128 5/1918 Seguin 123/75 CC 1,925,851 9/1933 Spencer.... 123/75 CC 2,242,538 5/1941 Naccache..... 123/75 CC 3,200,799 8/1965 Hammick..... 123/59 A 3,499,425 3/1970 Gommel 123/73 R Primary Examiner-Laurence M. Goodridge Assistant Examiner-Cort R. Flint Attorney-William R. Coffey et a1.

[5 7] ABSTRACT A 4-cycle, internal combustion engine comprising a crankcase and a pair of cylinders, each cylinder being open at one end into communication with the crankcase. Each cylinder is provided with an inlet port, and a crankshaft is disposed in the crankcase to be adjacent the open ends of the cylinders. A piston is reciprocably received in each cylinder to define therewithin a variable-volume combustion chamber, each piston being drivingly connected in a conventional manner to the crankshaft. A system is provided for admitting a gaseous mixture to the crankcase, and there is a passageway leading from the crankcase to each inlet port. valves are provided for respectively opening and closing the passageways, the valves being drivingly connected to and operated by the crankshaft. The valves are arranged and driven such that each passageway is opened only on alternate occasions when both pistons move toward the axis of the crankshaft. Thus, the pumping action of both pistons drives the gaseous mixture through the only open passageway into one of the two cylinders. The inlet port and an exhaust port is provided in the side wall of each cylinder at peripherally spaced apart points remote from the closed end of the cylinder. An auxiliary chamber in open communication with each exhaust port is provided, each said auxiliary chamber being provided with a port at a point remote from the point of communication of the chamber with the cylinder exhaust port. A butterfly valve is provided for dominating each auxiliary chamber port, the butterfly valve being drivingly connected to the crankshaft.

1 Claim, 14 Drawing Figures l 1 I I 3,756,206

Sept. 4, 1973 United States Patent 1m Gommel PATENIEDSEP 4am 3.756206 SHEET 3 0F 4 a 4 I J I FIRING ORDER- l'3-2-4 Fig.6

Fig.7

2- CYLINDER 4- CYLINDER 6- CYLlNDER FIRING ORDER- l-3-5-2-4-6 PATENTEUSEP 4 ma 3.756206 sumuom -YLIND Fig.9

2 3 Z-CYLINDER 4-CYLINDER FIRING ORDER l-2-3-4 Fig-l2 6- LJ NDER Fl G ORDER-l-2-3-4-5-6 5 6 Y 8 1 155.13 Q-CYLINDER Fl RIN G ORDER-i-2-4-3-5-6 8-7 ENGINE The primary object of the present invention is to provide an improved internal combustion engine of the reciprocating piston type, embodying many of the desirable characteristics of 2-cycle engines and of 4-cycle engines while eliminating undesirable characteristics of both types. Particularly, an object of the present invention is to provide a 4-cycle, internal combustion engine comprising a crankcase and a pair of cylinders opening away from the crankcase and being in communication therewith. A piston is reciprocably disposed in each cylinder to define therewithin a variable-volume combustion chamber. The two cylinders, which may be opposed or side-by-side, are provided with inlet ports, and there is a gaseous mixture carrying passageway leading from the crankcase to each inlet port. Means for admitting a gaseous mixture to the crankcase is provided. Valve means for opening and closing the passageways are provided, the valve means being arranged and driven such that each passageway is opened only on alternate occasions when both pistons move toward the axis of the crankshaft in the crankcase so that the gaseous mixture in the crankcase is pumped through the open passageway into its associated cylinder. In this manner, since the firing cycle of the two cylinders alternate, a two-piston volume charge of gaseous mixture is driven into each cylinder before its piston begins its compression stroke.

In the illustrative embodiment, the gaseous mixture is admitted into the crankcase by means including reed valves which open when the pressure in the crankcase is below the pressure of the source of gaseous mixture and which close when the pressure in the crankcase exceeds that of the gaseous mixture. The reeds open, therefore, when the pistons move away from the axis of the crankshaft and close when the pistons move toward the axis of the crankshaft. The crankshaft and the pistons and the connecting rods are proportioned and designed such that, at any time, the positions of the pistons respectively in the cylinders are substantially identical. That is, it is the simultaneous movement of the pistons away from the crankshaft which draws a gaseous mixture into the crankcase and toward the crankshaft which pumps two-piston volumes of gaseous mixture through the open passageway into one of the cylinders.

One important advantage of the engine of the present invention is that the inlet valve to each cylinder, i.e., the valve which opens and closes the passageway leading from the crankcase to the inlet port of each cylinder, is out of the high heat and high pressure area of the cylinder. Additionally, an exhaust chamber with a butterfly valve dominating its exhaust port is attached to each cylinder to be out of the high heat and high pressure area of the cylinder. This chamber between the exhaust port in the side wall of each cylinder and the butterfly valve permits the gases to burn completely at low heat and low pressure, thereby very significantly to reduce the temperature of the exhaust. This factor will, of course, reduce the pollution content of the exhaust and particularly the nitrous oxide content of the exhaust. It will be appreciated that a simple butterfly valve may not be air tight, and this is an advantage of my structure. Butterfly valves are inexpensive as compared to conventional poppet exhaust valve assemblies.

The engine of the present invention includes two cylinders or any multiple of two cylinders. The cylinders may be either opposed or side-by-side.

I refer to my US. Pat. No. 3,499,425 issued Mar. 10, 1970 and also disclosing an engine combining many of the desirable characteristics of 2-eycle and 4-cycle engmes.

To the accomplishment of the above and related objects, my invention may be embodied in the forms illustrated in the accompanying drawings, attention being called to the fact, however, that the drawings are illustrative only, and that change may be made in the specific constructions illustrated and described, so long as the scope of the appended claims is not violated.

In the drawings:

FIG. 1 is a sectional view taken along two planes perpendicular to the axis of the crankshaft and showing the opposed cylinder version of the engine of the present invention, the two planes being respectively on the centers of the two cylinders;

FIG. 2 is a sectional view taken along a plane perpendicular to the axis of the crankshaft and showing the side-by-side version of the engine of the present invention;

FIG. 3 is a fragmentary sectional view showing the inlet valve means which controls the flow of gaseous mixture from the crankcase alternately'to each side-byside cylinder;

FIGS. 4a and 4b are sectional views of the rotary valve member of FIG. 3 taken along the respective indicated section lines;

FIG. 5 is a diagrammatical view showing the manner in which the rotary valves and butterfly valves are drivingly connected to the crankshaft of the opposed cylinder engine of FIG. 1;

FIGS. 6-9 are diagrammatical views of a 2-cylinder, 4-cylinder, 6-cylinder and 8-cylinder engine with the cylinders side-by-side and showing the firing order of the cylinders; and

FIGS. 10-13 are diagrammatical views showing 2- cylinder, 4-cylinder, 6-cylinder and 8-cylinder engines of the opposed type and showing the firing orders of the cylinders.

Referring more particularly to FIG. 1, it will be seen that I have illustrated an engine of the internal combustion, reciprocating-piston type, indicated generally by the reference numeral 10, and comprising a cylinder block assembly 12 providing a generally cylindrically shaped crankcase 14 with opposed cylinders 16, 18 extending radially outwardly from the crankcase. A crankshaft 20 is disposed in the crankcase to extend axially therealong, and a piston 22, 24 is reciprocably received in each cylinder 16, 18 conventionally to define therewithin a variable-volume combustion chamber. Each piston 22, 24 is drivingly connected to the crankshaft in a conventional manner by a rod 26, 28. Each piston 22, 24 conventionally includes a piston head 30 and a skirt 32 depending from the head and extending toward the crankshaft 20 and a trunnion 34 extending diametrically across the skirt. Each rod 26, 28 has one end 36 journal mounted to the trunnion 34 of its associated piston and its other end 38 journal mounted on an eccentric bearing portion 40 of the crankshaft 20.

importantly, it will be appreciated that each piston 22, 24 also includes shield means 46 closing the space bounded by its piston head 30 and skirt 32, the shield means being formed to provide a cavity opening toward the crankshaft 20 and proportioned to provide clearance for the oscillation of the said one end 38 of the associated piston rod 26, 28. The purpose of this shield means 46 is to reduce the overall volume of the space within the crankcase 14 and pistons. This feature will be discussed in more detail as this description progresses.

Each cylinder l6, 18 is conventionally provided with a threaded spark plug opening 52 in its closed end. It will be appreciated that the illustrative engine is an air-cooled engine; but it will be understood that the basic engine may be either water cooled or air cooled, depending upon the particular application requirements.

The engine 10 is constructed such that a gaseous mixture is admitted to the crankcase 14 and then distributed to the cylinders 16, 18. For this reason, the volume of the space within the crankcase 14 is reduced by defining that space, where possible, by a wall such as indicated at 58 cylindrically formed about the rotational axis 60 of the crankshaft 20. Further, the crankshaft 20 is formed to provide eccentric bearing portions 40 on which the ends 38 of the rods 26, 28 are journal mounted and cylindrical counter balance portions concentric with the axis of the crankshaft 20 and of a diameter just slightly smaller than the internal diameter of the crankcase 14. The combined axial length of the concentric cylindrical portions of the crankshaft 20 are generally equal to the axial length of the crankcase 14 less the combined axial length of the bearing portions 40, thereby to reduce the volume of the gaseous mixture receiving space in the crankcase 14. In other words, the crankcase 14 is made generally cylindrical and the crankshaft is made concentrically cylindrical at all portions other than the eccentric bearing portions 40 and of a diameter such that it will occupy as much of the space of a crankcase as possible. This structural feature accompanied by the shield means 46 on each piston 22, 24 creates a very effective pump for driving the gaseous mixture admitted to the crankcase 14 alternately into the cylinders 16, 18 when the pistons 22, 24 simultaneously move toward the axis of the crankshaft 20. The crankshaft 20, connecting rods 26, 28 and cylinders 16, 18 are proportioned and designed such that, at any time, the positions of the pistons 22, 24, respectively, in the cylinders are substantially identical.

In this description and in the appended claims, the words gaseous mixture are intended to include air by itself or air mixed with the vapor of a fuel such as gasoline orveven a fuel vapor by itself. When fuel injection techniques are used, air by itself is admitted to the crankcase 14. In non-fuel injection engines, a mixture of air and gasoline vapors is admitted to the crankcase 14.

Means are provided for admitting a gaseous mixture to the crankcase 14, the illustrative means including an inlet conduit 62, a plate 63 providing a pair of ports 64 leading from the conduit 62 to a space 66 in the crankcase 14, and a pair of reed valves 65 for closing the ports 64. The reed valves 65 normally close the ports 64. The valves 65, which may conventionally be 0.008 to 0.012 inches thick, are arranged such that when the pressure in the crankcase 14 drops below the pressure of the gaseous mixture source, the valves open to admit the gaseous mixture into the crankcase and, when the pressure in the crankcase exceeds that of the gaseous mixture source, the valves close the ports 64. Particularly, the valves 65 open when the pistons 22, 24 move away from the crankshaft 20 and close when the pistons move toward the crankshaft. The pistons 22, 24 and the reed valves 65, therefore, act as a pump. Conventionally, curved stops 68 are provided for limiting the flexing of the reed valves.

Each cyinder 16, 18 is provided at peripherally spaced points remote from its closed end, with an inlet port or ports 70 and an exhaust port or ports 72. The arrangement of these ports 70, 72 is such that they are opened and closed by movement of the pistons 22, 24 in the cylinders 16, 18. The advantages of such a port arrangement are discussed in my said prior US. Pat. No. 3,499,425.

The cylinder block assembly 12 includes, with each cylinder 16, 18, a housing 74 including a port 76 therein providing an auxiliary chamber 78 in open communication with the cylinder exhaust port 72. It will be appreciated that the port 76 is at a point remote from the point of communication of the auxiliary chamber 78 with the exhaust port 72, and that the port 76 is dominated by a butterfly valve 80 carried on a shaft 82. The butterfly valve 80 may be rotatably or oscillably driven.

The cylinder block assembly 12 includes means providing a passageway 90, 92 leading from the crankcase 14 to each inlet port 70. In the illustrative engine 10, adjacent each inlet port 70 is a fuel injector fitting or nozzle 94, 96 which may be conventionally connected to a source of fuel vapor. in such a case, when air is pumped from the crankcase 14 through the passageway 90, 92, the fuel vapor is mixed with the air and admitted to the cylinder 16, 18 through the port 70.

Then, a rotary valve 100, 102 extends across each passageway 90, 92 to open and close that passageway, each rotary valve being received in a bore 104, 106 which is parallel with the axis 60 of the crankshaft 20. Each rotary valve 100, 102 is provided with a cutout 110, 112 on its periphery such that each passageway 90, 92 is opened during a portion of each revolution of each valve. The valves 100, 102 are both drivingly connected to the crankshaft 20 and are arranged so that their respective passageways 90, 92 are opened only on alternate occasions when both pistons 22, 24 move toward the axis 60 of the crankshaft 20. That is, each time the pistons 22, 24 move toward the axis 60, the fuel mixture in the crankcase 14 is pumped through the opened passageway or 92 into the cylinder 16 or 18 to be compressed. In preferred embodiments, each passageway 90, 92 remains open throughout the period when its associated inlet port 70 is opened by the piston 22, 24. The manner in which the rotary valves 100, 102 may be driven to open each passageway 90, 92 only on alternate occasions when both pistons 22, 24 are moving toward the axis 60 will be discussed in conjunction with FIG. 5.

Turning now to FIGS. 2, 3, 4a and 4b, a discussion of my side-by-side version of the engine of the present invention will be discussed, like reference numerals indicating like parts. This engine 10' is arranged such that its cylinders 16, 18 are disposed side-by-side with their axes lying in a plane which is radial to the axis 60 of the crankshaft 20. The two cylinders 16, 18 open into the common crankcase 14' which is generally cylindrically formed for the reasons discussed above.

The two passageways 90, 92 leading to the inlet ports 70 from the crankcase 14' are alternately opened and closed by means of a single rotary valve member 130 which extends parallel to the axis of the crankshaft and across both passageways. This illustrative valve member 130 is provided with a cutout 132 associated with the passageway 90 and a diametrically opposite cutout 134 associated with the passageway 92. The valve member 130 is driven from the crankshaft and the cutouts 132, 134 are positioned such that each passageway 90, 92 is opened only on alternate occasions when the two pistons 22, 24 move toward the axis 60 of the crankshaft 20. The eccentric bearing portions 40 associated with the pistons 22, 24 are identically disposed so that the two pistons move simultaneously together toward and away from the axis of the crankshaft 20.

Turning now to FIG. 5, it will be seen that I have shown diagrammatically the rotary valves 100, 102, the crankshaft 20 and the shafts 82 on which the butterfly valves 80 are carried. Illustratively, an idler gear 146 is used drivingly to connect a gear 147 on the crankshaft 20 to a gear 148 on the rotary valve 100. The gear 148 is engaged with a gear 150 on the rotary valve 102. A cogwheel 152 is mounted on the rotary valve 100, a cogwheel 154 is mounted on the shaft 82 and a cogbelt 156 drivingly connects the shaft 82 associated with the cylinder 16 to the rotary valve 100. Similarly, a co'gwheel 158 is mounted on the valve 102, a cogwheel 160 is mounted on the other shaft 82 associated with the cylinder 18, and a cogbelt 162 is trained about these cogwheels. It will be appreciated that this drive system is merely illustrative and that other forms of drive systems may be used.

In some embodiments of the present engine, the valves 100, 102 may be rotated at' one-half the speed of the crankshaft 20 while the two shafts 82 are rotated at one-fourth the speed of the crankshaft.

With the above description in mind, the 4cycle operation of the engine of the present invention will be discussed. The firing sequence for the cylinder 16 with its piston 22 is as follows:

On the power stroke the piston 22 moves toward the axis 60 and the exhaust goes out the exhaust port 72 and the port 76 which is opened by the butterfly valve 80. The exhaust continues to go out the port 72 until the piston 22 moves back up to close port 72. The inlet valve 100 is closed to block the passage 90 during the power stroke of the piston 22 so that the fuel mixture cannot be pumped from the crankcase 14 into the cylinder 16 during the power stroke. The piston 22 then goes up again. The piston 22 then goes down again or toward the axis 60. During this stroke, the valve 100 is in a position such that the passageway 90 is open and the butterfly valve 80 is closing the port 76. The twopiston volumes of air resulting from movement of both pistons 22, 24 toward the axis 60 pumps the gaseous mixture through the passageway 90 into the cylinder 16. The piston 22 then goes up or away from the axis 60 on its compression stroke to compress the gaseous mixture for firing.

Recapitulating, the engine of the present invention is a 4-cycle engine with each piston 22, 24 having four strokes including stroke N o. 1 which is a power stroke at the bottom of which the exhaust leaves the cylinder through the exhaust port and the open butterfly valve; stroke No. 2 is the piston moving up again in the cylinder; stroke No. 3 is the piston moving downwardly again in the cylinder to pump (together with its associated piston) the gaseous mixture from the crankcase 14 into the cylinder; and stroke No. 4 is a compression stroke.

The cylinder 18 has the same firing sequence as the cylinder 16 except that the cylinder 18 is on opposite cycles. That is, when the piston 22 is on its power stroke, the piston 22 and the piston 24 pump the gaseous mixture from the crankcase .14 through the passageway 92 into the cylinder 18 for compression and firing. When the piston 24 is on its power stroke, the piston 24 and the piston 22 pump the gaseous mixture from the crankcase 14 through the passageway 90 into the cylinder 16 for compression and firing. Thus, when one piston 22, 24 is on its power stroke, the other piston is on its intake stroke, and when one piston is on its compression stroke, the other piston is on its stroke which is conventionally referred to as the exhaust stroke in the Otto cycle engine.

When the engine of the present invention is running at high speeds, e.g., over 4,000 rpm, the butterfly exhaust valves may be eliminated or held steadily open.

Modern automobile engines often include means for recycling the exhaust through the intake to reduce pollution. Some other modern automobile engines supply compressed air to the exhaust to burn the exhaust gases at low pressure to reduce pollution. The engine of the present invention obtains this feature of burning the exhaust gases at low pressure to reduce pollution. Particularly, the fresh gases pumped into the cylinders 16, 18 will mix with and burn the exhaust gases trapped in the chambers 78, and this burning will take place at low pressure. The effect of this is to reduce the pollution content of the exhaust and particularly the unburned hydrocarbon content of the exhaust.

The exhaust of the engine of the present invention is extremely cool. An engine constructed in accordance with the present invention can be operated and a person can place his hand in the output side of the butterfly exhaust valve and not be burned by the exhaust itself. This, of course, reduces the nitrous oxide content of the exhaust which is a pollution factor producing smog and eye irritation.

Butterfly valves of the engine of the present invention may rotate at one-fourth, three-fourths or 1% the speed of the crankshaft 20. The butterfly valves may be oxcillated by a crank running at one-half the speed of the crankshaft 20.

An important advantage of the engine of the present invention is that the fuel injection fittings or nozzles 94, 96, if they are used, may be out of the high heat and high pressure zones of the cylinders 16, 18.

The 4-cycle engine of the present invention has approximately the same weight as comparable 2-cycle engines, approximately the same manufacturing costs as comparable 2-cycle engines and approximately the same height as comparable 2-cycle or I. head engines.

Turning now to FIGS. 6-9, it will be seen that I have shown, by diagrammatical views, how a 2-cylinder, 4- cylinder, 6-cylinder or S-cylinder engine may be constructed using the side-by-side arrangement discussed in conjunction with FIG. 2 as well as a proposed firing order for such 4-cyclinder, 8-cylinder and 8 cylinder engines. Then, in FIGS. 10-13, I have provided diagrammatical views showing how a Z-cylinder, 4- cylinder, 6-cylinder or S-cylinder engine may be constructed using the opposed cylinder concept discussed in conjunction with FIG. 1 as well as a proposed firing order for the 4-cylinder, 6-cylinder and 8-cylinder engine.

lt will be appreciated that an extremely compact and relatively short engine may be provided by constructing the engines of FIGS. 10-13 as horizontally opposed engines.

Lubrication may be provided by making oil with the fuel mixture going into the crankcase or by positive pressure pumping of oil to the bearings and pistons.

What is claimed is:

l. A 4-cycle, internal combustion engine comprising a pair of cylinders, each cylinder being open at one end and provided, at peripherally spaced points remote from the other end of said cylinder, with an inlet port and an exhaust port in its side walls, a crankshaft arranged adjacent the open ends of said cylinders, a piston reciprocably received in each said cylinder and defining therewithin a variable-volume combustion chamber, means extending through the open end of each said cylinder and drivingly connecting the piston therein to said crankshaft, the last said means and said crankshaft being arranged so taht said pistons move simultaneously toward and away from the axis of said crankshaft, each said piston being movable in its associated cylinder past its said ports to close and open same during each revolution of said crankshaft, a crankcase for said crankshaft, means for admitting a gaseous mixture to said crankcase, means providing a passageway from said crankcase to the inlet port of each said cylinder, and valve means for opening and closing said passageways, said valve meeans being drivingly connected to and operated by said crankshaft, and said valve means being arranged alternately to open said passageways only on occasions when said pistons move toward the axis of said crankshaft, said crankcase being generally cylindrically shaped about the axis of said crankshaft, said crankshaft being formed to provide eccentric bearing portions on which said means for drivingly connecting said pistons to said crankshaft are journalled and, on opposite ends of said bearing portions, counter balance portions which are fully cylindrical and concentric with the axis of said crankshaft and of a diameter slightly smaller than the internal diameter of said crankcase, the combined axial length of said counter balance portions being generally equal to the axial length of said crankcase less the combined axial lengths of said bearing portions, thereby to reduce the volume of the gaseous mixture receiving space in the crankcase to increase the efficiency with which the piston movement pumps said gaseous mixture to said cylinders, said means for admitting a gaseous mixture to said crankcase including reed valve means arranged to open when the pressure in said crankcase drops below the pressure of the gaseous mixture source and to close when the pressure in said crankcase exceeds the pressure of said source, said first mentioned valve means including rotary valve means, said rotary valve means including one or more valve members extending parallel to the axis of said crankshaft and across each said passageway, each of said valve members being formed to close its associated passageway during a portion of each revolution and to open it during the rest of each revolution throughout the period when its associated inlet port is opened by its associated piston, means defining an auxiliary chamber in open communication with the exhaust port of each said cylinder, each said auxiliary chamber being provided with a port at a point remote from the point of communication of said auxiliary chamber with said exhaust port, butterfly valve means dominating said auxiliary chamber port, and means driven from said crankshaft for opening said butterfly valve means only on the occasions when its associated cylinder exhaust port is opened by the piston therein moving downwardly on its power stroke.

i i i t 

1. A 4-cycle, internal combustion engine comprising a pair of cylinders, each cylinder being open at one end and provided, at peripherally spaced points remote from the other end of said cylinder, with an inlet port and an exhaust port in its side walls, a crankshaft arranged adjacent the open ends of said cylinders, a piston reciprocably received in each said cylinder and defining therewithin a variable-volume combustion chamber, means extending through the open end of each said cylinder and drivingly connecting the piston therein to said crankshaft, the last said means and said crankshaft being arranged so taht said pistons move simultaneously toward and away from the axis of said crankshaft, each said piston being movable in its associated cylinder past its said ports to close and open same during each revolution of said crankshaft, a crankcase for said crankshaft, means for admitting a gaseous mixture to said crankcase, means providing a passageway from said crankcase to the inlet port of each said cylinder, and valve means for opening and closing said passageways, said valve meeans being drivingly connected to and operated by said crankshaft, and said valve means being arranged alternately to open said passageways only on occasions when said pistons move toward the axis of said crankshaft, said crankcase being generally cylindrically shaped about the axis of said crankshaft, said crankshaft being formed to provide eccentric bearing portions on which said means for drivingly connecting said pistons to said crankshaft are journalled and, on opposite ends of said bearing portions, counter balance portions which are fully cylindrical and concentric with the axis of said crankshaft and of a diameter slightly smaller than the internal diameter of said crankcase, the combined axial length of said counter balance portions being generally equal to the axial length of said crankcase less the combined axial lengths of said bearing portions, thereby to reduce the volume of the gaseous mixture receiving space in the crankcase to increase the efficiency with which the piston movement pumps said gaseous mixture to said cylinders, said means for admitting a gaseous mixture to said crankcase including reed valve means arranged to open when the pressure in said crankcase drops below the pressure of the gaseous mixture source and to close when the pressure in said crankcase exceeds the pressure of said source, said first mentioned valve means including rotary valve means, said rotary valve means including one or more valve members extending parallel to the axis of said crankshaft and across each said passageway, each of said valve members being formed to close its associated passageway during a portion of each revolution and to open it during the rest of each revolution throughout the period when its associated inlet port is opened by its associated piston, means defining an auxiliary chamber in open communication with the exhaust port of each said cylinder, each said auxiliary chamber being provided with a port at a point remote from the point of communication of said auxiliary chamber with said exhaust port, butterfly valve means dominating said auxiliary chamber port, and means driven from said crankshaft for opening said butterfly valve means only on the occasions when its associated cylinder exhaust port is opened by the piston therein moving downwardly on its power stroke. 